Recently, magneto-optical disks where information can be recorded, reproduced and erased have been developed as substitutes for the Read-Only type optical disks such as compact disks. As an example of information reproducing device, description will be made hereinbelow of a magneto-optical disk device capable of recording, reproducing and erasing information on a magneto-optical disk.
As illustrated in FIG. 28(a), a typical magneto-optical disk is constituted of disk substrate 2804 and a recording magnetic film 2805 formed on the disk substrate 2804. The recording magnetic film 2805 is formed such that its axis of easy magnetization is perpendicular to the film surface thereof, and is initialized such that the direction of magnetization indicated by an arrow A or an arrow B within the film shown in FIG. 28(a) is preliminary set in a fixed direction (for example, shown by the arrow A in FIG. 28(a)).
During recording, a laser beam 2803 is projected from a semiconductor laser 2801, converged by an objective lens 2802 so as to have a diameter of approximately 1 .mu.m and is irradiated on the recording magnetic film 2805. At this time, the intensity of the laser beam 2803 is controlled according to a recording signal 2807 (see FIG. 28(b)) corresponding to the information to be recorded. When the recording signal 2807 is in the high level and thereby the intensity of the laser beam 2803 is strong, the temperature of the area illuminated by the strong laser beam 2803 rises locally, goes above the Curie point, and the coercive force of the area illuminated significantly lowers. An external magnetic field 2806 is applied at the same time that the laser beam 2803 is irradiated. As a result, the direction of magnetization A in the area where the coercive force lowered is inverted and frozen in the same direction of magnetization B as the external magnetic field 2806 thereby permitting information corresponding to the recording signal 2807 to be recorded on the recording magnetic film 2805. Hereinafter, parts where high level recording signals 2807 were recorded as described above and where the direction of magnetization is B will be referred to as marks 2809, and parts where low level recording signals 2807 were recorded and where the direction of magnetization is A will be referred as non-marks 2810.
Information recorded on the recording magnetic film 2805 is erased by inverting the direction of the external magnetic field 2806 and following a method similar to the one used for recording. The direction of magnetization is restored to its original direction of initialization, i.e. the direction of magnetization A in FIG. 28(a), and the recorded information is erased. Marks 2809 thus become non-existent in the erased part.
In the present example, the light modulation method is adopted, i.e. recording is executed by modulating the intensity of the laser beam 2803 in accordance with the recording signal 2807, and applying an external magnetic field 2806 of a constant intensity. However, the magnetic modulation method may as well be adopted and recording can be executed by making the intensity of the laser beam 2803 constant and modulating the direction of the external magnetic field 2806 in accordance with the recording signal 2807.
The disk substrate 2804 mentioned earlier is made of glass, plastic or other material, and lands and pits 2808 are preliminary etched thereon, as shown in FIG. 28(a). The lands and pits 2808 represent address information indicating the addresses of tracks and sectors. The above address information is preliminary etched onto the disk substrate 2804 during the manufacturing stage of the magneto-optical disk according to a fixed format. Hence, the lands and pits 2808 cannot be recorded or erased thereafter. Hereinafter, parts where a plurality of lands and pits 2808 are formed in a group will be referred to as pre-formatted sections 3003. Information is recorded and erased in areas other than the pre-formatted sections 3003. These areas will be referred to hereinafter as MO (magneto-optical) sections 3002. Pre-formatted sections 3003 and MO sections 3002 are usually accommodated alternately to form a track 3005 in a spiral shape or in the shape of concentric circles, as illustrated in FIG. 30. A sector 3004 is constituted by a pair composed of a pre-formatted section 3003 and MO section 3002.
As illustrated in FIG. 30, a magneto-optical disk 3001 comprises a plurality of sectors 3004 formed on the track 3005, each sector 3004 being provided with address information. Information is recorded, reproduced and erased sector 3004 by sector 3004.
As illustrated in FIG. 31, the pre-formatted sections 3003 of the tracks 3005 are arranged such that either the land or the pit that compose one land and pit 2808 shown in FIG. 28(a) form a mark 2811, and such that the other component of the land and pit 2808 form a non-mark 2812. Marks 2809 and non-marks 2810 are recorded in the MO section 3002 in response to MO signals as described earlier.
When reproduction is performed on the magneto-optical disk 3001, the laser beam 2803 is projected from the semiconductor laser 2801, is converged by the objective lens 2802 so as to have a diameter of 1 .mu.m and is irradiated upon the recording magnetic film 2805, as illustrated in FIG. 29(a). Here, the intensity of the laser beam 2803 is weaker when information is reproduced than when information is recorded or erased. The laser beam 2803 is a linearly polarized light and its plane of polarization is rotated as the laser beam 2803 passes through or is reflected by the recording magnetic film 2805 due to the Faraday effect or the Kerr effect. The plane of polarization of the laser beam 2803 is rotated in mutually opposite directions depending on whether the laser beam 2803 is irradiated on a mark 2809 or a non-mark 2810. Reproduction of recorded information is performed by detecting the difference in polarization direction. Accordingly, two types of reproduced signals S1 and S2, shown by (b) and (c) in FIG. 29, are generated.
The reproduction optical system employed for producing the reproduced signals S1 and S2 will be discussed briefly hereinbelow. As illustrated in FIG. 32, a reflected light 3201 coming from the magneto-optical disk 3001 is directed toward a PBS (analyzer) 3202 where it is split according to its polarization direction through the Kerr effect. Two detected lights 3210 and 3211 that were separated in the PBS 3202 are respectively directed toward photodetectors 3203 and 3204 where they are converted into electric signals that vary according to the respective intensities of the detected lights 3210 and 3211, and released as reproduced signals S1 and S2. As it will be covered in details later, the signals from the pre-formatted section 3003 and the MO section 3002 are obtained separately by determining the sum and the difference of the reproduced signals S1 and S2. In addition, the marks 2809 and the non-marks 2810 may be reproduced separately through the signals of the MO section 3002 thereby enabling the information recorded on the recording magnetic film 2805 to be reproduced.
Suppose that a represents the vector of a reflected light from a non-mark 2810 (direction of magnetization A) of the MO section 3002, and .beta. represents the vector of a reflected light from a mark 2809 (direction of magnetization B) of the MO section 3002. The reflected light vectors .alpha. and .beta. are rotated in opposite directions by an angle corresponding to the rotation angle of their respective plane of polarization, as illustrated in FIG. 33. The X direction components and Y direction components of the reflected light vectors .alpha. and .beta. are detected in the PBS 3202 that transmits light having a X or Y polarization direction. These two polarization directions X and Y form a right angle.
Geometrical explanation will be made hereinbelow. The reflected light vector .alpha. is projected in the polarization direction X and the polarization direction Y thereby producing detected light vectors .alpha..sub.X and .beta..sub.Y. Similarly, the reflected light vector .beta. is projected in the polarization direction X and the polarization direction Y thereby producing detected light vectors .beta..sub.X and .beta..sub.Y. The magnitudes of detected light vectors .alpha..sub.X and .beta..sub.X correspond to the reproduced signal S1 and the magnitudes of the detected light vectors .alpha..sub.Y and .beta..sub.Y corresponds to the reproduced signal S2. Further, the detected light vectors .alpha..sub.X and .beta..sub.X correspond to the detected light 3210 shown in FIG. 32, and the detected light vectors .alpha..sub.Y and .beta..sub.Y correspond to the detected light 3211.
Assume, as illustrated in FIG. 33, the high level of the reproduced signal S1 corresponds to a non-mark 2810 and the low level of the reproduced signal S1 corresponds to a mark 2809. Here, the high level of the reproduced signal S2 corresponds to a mark 2809 and its low level to a non-mark 2810. The polarity of the reproduced signal S1 and the polarity of the reproduced signal S2 are thus opposite. The reproduced signals S1 and S2 are then fed into a differential amplifier where the difference of the reproduced signals S1 and S2 is determined and the reproduced signals S1 and S2 are amplified and thereby their S/N is improved, and information is reproduced.
The reproduced signals S1 and S2 obtained from the pre-formatted sections 3003 will be described hereinbelow with reference to FIG. 34. As there is no recording nor erasing operation taking place in the pre-formatted sections 3003, the direction of magnetization therein coincides with the direction A only. When the laser beam 2803 is irradiated on a pre-formatted section 3003, the shape of the marks 2811 and non-marks 2812, i.e. the lands and pits 2808, causes the laser beam 2803 to be diffracted. As a result, a long reflected light vector .delta. or a short reflected light vector .epsilon. is produced according to the land or pit 2808, as illustrated in FIG. 34. Namely, a long reflected light vector .delta. is produced when for example a non-mark 2812 is reproduced, and a short reflected light vector s is produced when a mark 2811 is reproduced.
A detected light vector .delta..sub.X and a detected light vector .delta..sub.Y are produced by projecting the reflected light vector .delta. in the polarization direction X and in the polarization direction Y of the PBS 3202. Similarly, a detected light vector .epsilon..sub.X and a detected light vector .epsilon..sub.Y are produced by projecting the reflected light vector .epsilon. in the polarization direction X and in the polarization direction Y of the PBS 3202. The magnitudes of the detected light vector .delta..sub.X and of the detected light vector .epsilon..sub.X correspond to the reproduced signal S1, and the magnitudes of the detected light vector .delta..sub.Y and of the detected light vector .epsilon..sub.Y correspond to the reproduced signal S2. The high level of the reproduced signal S1 and and the high level of the reproduced signal S2 both correspond to a non-mark 2812 of the lands and pits 2808, the low level of the reproduced signal S1 and the low level of reproduced signal S2 correspond to marks 2811. Consequently, as illustrated in FIGS. 29(b) and (c), the reproduced signals S1 and S2 have the same polarity for the pre-formatted section 3003 while they have mutually inverted polarities for the MO section 3002.
As a result, when for example determining the difference between the reproduced signal S1 and the reproduced signal S2 in a differential amplifier 10 shown in FIG. 35, an analog signal will be obtained only for information of the MO section 3002. Meanwhile, when determining the sum of the reproduced signals S1 and S2 in a summing amplifier not shown, an analog signal is obtained only for information of the pre-formatted section 3003. In such a fashion the S/N may be improved.
A binary conversion circuit adapted for information from the MO section 3002, will be described hereinbelow as an example of circuit for converting the analog signals obtained as mentioned above into binary signals. As illustrated in FIG. 35, the analog signal that was released by the differential amplifier 10, (i.e. that was reproduced from the MO section 3002), is fed into a differentiating circuit 11, the non-inverting input terminal of a comparator 15 and a reference voltage generator 12. The analog signal is differentiated in the differentiating circuit 11 and the resulting differentiated signal is compared with a ground potential in a comparator 13. The comparator 13 subsequently releases a zero-cross signal, i.e. a signal that goes in the high level and in the low level when the differentiated signal crosses its zero level, that is fed into a gate circuit 14.
Meanwhile, the reference voltage generator 12 generates a reference voltage in accordance with the analog signal supplied from the differential amplifier 10, and sends this reference voltage into the inverting input terminal of a comparator 15. In the comparator 15, the analog signal supplied from the differential amplifier 10 is compared with the above reference voltage and is converted into a binary signal, and a gate signal is generated. The gate signal is fed into the gate circuit 14. A reproduced data signal is generated in the gate circuit 14 based on the zero-cross signal and the gate signal, as it will be discussed later. Analog signals reproduced from pre-formatted sections 3003, are converted into binary signals in a circuit having a configuration similar to the one illustrated in FIG. 35 except that the differential amplifier 10 is replaced by a summing amplifier.
Waveforms of signals generated in the different sections of the binary conversion circuit shown in FIG. 35 will be described hereinbelow with reference to FIG. 36. Here, it is assumed that modulated data as shown by (a) in FIG. 36 was modulated and generated through for example a 2-7 modulation method (to be covered in detail below). In this case, the mark 2809 of the MO section 3002 (or mark 2811 of the land and pit 2808) is recorded such that the center thereof coincides with the binary code "1" of the modulated data, as illustrated by (b) in FIG. 36. A mark 2809 is reproduced by means of a laser spot 2701, and the analog signal reproduced from the MO section 3002 as shown by (c) in FIG. 36, is obtained by determining the difference between the reproduced signals S1 and S2. When the mark 2811 is reproduced, the analog signal reproduced from the pre-formatted section 3003 is obtained by determining the sum of the reproduced signals S1 and S2. The analog signal obtained as described above, is differentiated in the differentiating circuit 11 and the differentiated signal as shown by (d) in FIG. 36 is obtained. The differentiated signal is fed into the comparator 13 that releases the zero-cross signal signal as shown by (e) in FIG. 36.
The analog signal shown by (c) in FIG. 36 is converted into the binary signal in the comparator 15 and the gate signal as shown by (f) in FIG. 36 is generated. The gate signal is then fed into the gate circuit 14. The gate circuit 14 releases a high level ("1") signal when the zero-cross signal falls while the gate signal is in the high level and releases a low level ("0") signal simultaneously with the change of the gate signal to the low level. As a result, the reproduced data signal as shown by (g) in FIG. 36 is released from the gate circuit 14. Based on the reproduced data signal, reproduced data which binary code corresponds to "1" only when the reproduced data signal rises, can be obtained.
However, in sections such as sections C and D shown in FIG. 36(b) where the interval between adjacent marks 2809 (or 2811) is narrow, in other words in parts where the frequency of the analog signal shown by (c) in FIG. 36 is relatively high, the peak-to-peak value of the analog signal is small. Therefore, when the analog signal is converted into a binary signal in the binary conversion circuit such as shown ill FIG. 35, the gate signal might stay in the high level in sections where it should drop to the low level, such as for example sections C and D of (f) in FIG. 36.
When, as described above, the gate signal does not drop to the low level in sections where it is supposed to, the reproduced data signal (shown by (g) in FIG. 36) consequently does not drop to the low level in sections where it should do so. Consequently, as shown by C.sub.1, C.sub.2, D.sub.1 and D.sub.2 in FIG. 36(h), the binary code of the reproduced data coincides with "0"where it should be "1" in order to correspond to the modulated data shown by (a) in FIG. 36. A conventional magneto-optical disk device thus presents a drawback that reproduction errors occur.
In addition, when variations occurred in the upper limit level and lower limit level of the analog signal shown by (c) in FIG. 36 due to irregularities in the reflectance on the magneto-optical disk 3001, with a conventional magneto-optical disk device, the gate signal shown by (f) in FIG. 36 becomes even more unreliable causing the occurrence of reproduction errors to increase.